What affects the color differences in Hydrangeas?
Bigleaf hydrangea (Hydrangea macrophylla) are renowned for large, rounded flower clusters whose color can change depending on soil chemistry. Bigleaf hydrangea grown in acidic soil tend to produce blue flowers while those grown in alkaline soil tend to produce pink ones. Testing your soil’s pH is the only way to know how it is likely to affect your hydrangea’s blooms. It’s also the only way to know how to treat your soil if you wish to influence your bigleaf hydrangea’s bloom color.
You can test your soil with a home pH soil meter or submit a soil sample to your local extension office for a more detailed analysis. A soil pH around 7 is neutral; the farther below 7 your pH measure, the more acidic your soil and the bluer your hydrangea blossoms; the farther above 7, the more alkaline your soil and the pinker your hydrangea blossoms. Always test your soil BEFORE attempting to chemically alter it and be patient once you do - it may take two or three seasons for each plant to firmly establish its flower color!
To encourage blue flowers:
- Acidify your soil by adding garden sulfur or potash.
- Adding aluminum sulfate to the soil may help speed the bluing process, but beware that adding too much is dangerous to the plant as it can seriously damage roots and leaves.
- Avoid use of super phosphates.
- Use collected rainwater for irrigating your hydrangeas, rather than tap water, which often contains lime deposits.
- Make sure there is adequate iron available in the soil as it plays an important role in the plant’s take up of certain nutrients, such as aluminum (the chemical responsible for making hydrangea flowers blue).
- The dry, papery spent flower heads of blue-hued hydrangeas contain aluminum which can be recycled back into the soil if you allow them to decompose on the ground around the base of your plants.
To encourage pink flowers:
- Add phosphorus to the soil to make it more alkaline.
- Plant near a cement foundation where free calcium is likely available in the soil.